Youngjo Tak

Fabrication of ZnO/CdS core/shell nanowire arrays for efficient solar energy conversion

ZnO/CdS core/shell nanowire heterostructure arrays are fabricated by a two-step chemical solution method for use in semiconductor-sensitized photoelectrochemical cells (PECs). The successive ion layer adsorption and reaction (SILAR) shows a remarkable controllability of CdS shell thickness, which affects visible-light absorption properties and PEC performances of the heterostructures. The cell has a high short-circuit photocurrent density of 7.23 mA/cm2 with a power conversion efficiency of 3.53% under AM 1.5G illumination at 100 mW/cm2. These results demonstrate that the ZnO/CdS core/shell heterostructure nanowire arrays can provide a facile and compatible frame for the potential applications in nanowire-based solar cells.

Novel nanowire array based highly efficient quantum dot sensitized solar cell

In this communication, a novel CdSe/CdS/ZnO nanowire array fabricated by a 3-step solution-based method was used as a photoanode of a quantum dot sensitized solar cell, which generated a maximum power conversion efficiency of 4.15%.

Characterization of ZnO nanorod arrays fabricated on Si wafers using a low-temperature synthesis method

ZnO nanorod arrays fabricated on ZnO buffer layers on Si wafers were grown using a low-temperature solution method and were characterized by various techniques. Buffer layers were prepared using metal organic chemical vapor deposition and a sputter-oxidation method. Aligned ZnO nanorods were deposited at 90°C on the substrates by a hydrothermal treatment using a zinc salt and aqueous ammonia solution. The ZnO nanorod arrays were characterized by scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. The as-grown ZnO nanorod arrays exhibited broad deep-level emission centered at ∼564nm. The intensity of the deep-level emission decreased and band edge emission centered at 379nm appeared after air annealing. Samples annealed in hydrogen showed only band edge emission.

Synthesis and Characterization of ZnO Nanowire–CdO Composite Nanostructures

ZnO nanowire–CdO composite nanostructures were fabricated by a simple two-step process involving ammonia solution method and thermal evaporation. First, ZnO nanowires (NWs) were grown on Si substrate by aqueous ammonia solution method and then CdO was deposited on these ZnO NWs by thermal evaporation of cadmium chloride powder. The surface morphology and structure of the synthesized composite structures were analyzed by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The optical absorbance spectrum showed that ZnO NW–CdO composites can absorb light up to 550 nm. The photoluminescence spectrum of the composite structure does not show any CdO-related emission peak and also there was no band gap modification of ZnO due to CdO. The photocurrent measurements showed that ZnO NW–CdO composite structures have better photocurrent when compared with the bare ZnO NWs.

Atomically abrupt heteronanojunction of ZnO nanorods on SiC nanowires prepared by a two-step process

A two-step process was developed to prepare a direct heteronanojunction of ZnO nanorods on SiC nanowires, using a simple heating method and metal?organic chemical vapour deposition (MOCVD). First, an SiC nanowire substrate was prepared by heating NiO catalysed Si wafer at 1050??C. Subsequently, diethylzinc was used as metal?organic source to grow ZnO nanorods on SiC nanowires at 450??C. High-resolution TEM images showed that the heteronanojunction has an atomically abrupt interface with no interfacial layers formed between ZnO nanorods and SiC nanowires. The epitaxial relationship between ZnO nanorods and SiC nanowires was ZnO[0001] ? SiC[111].

LOW-TEMPERATURE SYNTHESIZED ZnO NANONEEDLES: XPS AND PL ANALYSIS

Evolution of morphology change was investigated for ZnO nanoneedle array grown by low-temperature MOCVD. Well-aligned ZnO nanoneedle array was deposited on the ZnO buffer-film/Si substrate at temperatures below 500°C. A rod-shaped ZnO nanowire in the initial growth stage changed into needle-shaped as the deposition proceeds. ZnO nanoneedle array deposited on the annealed buffer-film showed better alignment compared to that deposited on the as-grown film. XPS analysis showed that Zn 2p peak has a single binding energy state of a stoichiometric Zn–O bond while O 1 s peak has three different chemical binding states. Highly crystalline ZnO nanoneedle array showed a strong bandedge emission at 380 nm in photoluminescence measurements.

Growth of Aligned ZnO Nanorods on Pt Buffer Layer Coated Silicon Substrates Using Metallorganic Chemical Vapor Deposition

A high density of aligned ZnO nanorods was grown on Pt buffer layer coated Si substrates. Ultrathin Pt buffer layers were deposited on Si(111). The ZnO nanorods were synthesized by metallorganic chemical vapor deposition using diethylzinc and oxygen. The optimum growth temperature was 450°C to obtain aligned ZnO nanorods. The grown ZnO nanorods had single-crystalline atomic structure and pure compositions without any impurities. The nanorods showed a strong near-band-edge PL emission at 3.27 eV with no significant deep-level emission peaks. The morphology of ZnO nanorods grown on Pt buffer layer was compared with other buffer layers such as atomic layer deposited ZnO and Zr metal buffer film.

Novel heterostructure of CdS nanoparticle/WO3 nanowhisker: Synthesis and photocatalytic properties

A novel heterostructure of CdS nanoparticles/WO3 nanowhiskers was synthesized using a simple two-step process; thermal evaporation and chemical bath deposition. First, WO3 nanowhiskers (NWs) were grown on a tungsten substrate by thermal evaporation of WO3 powder in a tube furnace at 1050°C. Sequentially, CdS nanoparticles (NPs) were deposited on WO3 nanowhiskers by chemical bath deposition. CdS nanoparticles modified WO3 nanowhiskers showed enhanced visible light absorption compared to bare WO3 nanowhiskers. The photocatalytic activity was studied by the photodegradation of methylene blue. CdS NP/WO3 NW heterostructures showed remarkably enhanced photodecomposition efficiencies compared to bare WO3 nanowhiskers.

ZrO2-COATED SiC NANOWIRES PREPARED BY PLASMA-ENHANCED ATOMIC LAYER CHEMICAL VAPOR DEPOSITION

Plasma-enhanced atomic layer deposition (PE-ALCVD) of ZrO2 was performed to coat SiC nanowires and prepare a SiC-ZrO2 core-shell nanowire structure. Zirconium tertiary butoxide (ZTB) and hydrogen plasma pulse cycles were used to grow ZrO2 films. The growth temperature of ZrO2 PE-ALCVD was 150°C with a growth rate of 1.3 A/cycle. SEM and TEM images showed uniform coating of SiC nanowires with ZrO2. The thickness of ZrO2 coat layer could be controlled by the total number of the pulse cycles. After being annealed at 900°C, a polycrystalline structure of ZrO2 layer was observed.

Effect of the cyclic delivery of (hexafluoroacetylacetonate)Cu(I) ×(3,3-dimethyl-1-butene) pulse and Ar purge gas on the low temperature copper metalorganic chemical vapor deposition

Cu thin films were deposited on the sputter-deposited Ta/Si substrate using cycles of alternate supply of (hexafluoroacetylacetonate)Cu(I)(3,3-dimethyl-1-butene) pulse and argon purge gas. The growth temperature was as low as 70 °C and 200–1000 cycles were performed to deposit films in pulsed-metalorganic chemical vapor deposition (MOCVD) with a growth rate of ∼0.75 A/cycle. Cross-sectional scanning electron microscopy of the pulsed-MOCVD Cu films showed continuous and uniform films, while the MOCVD Cu films showed void formations. Also, x-ray diffraction patterns of the Cu films showed the preferential crystallographic orientation of the (111) plane. The Cu films grown by MOCVD and pulsed MOCVD, both contained oxygen (O), carbon (C), and fluorine (F) as impurities. The impurities of C and F atoms were hard to detect in the bulk films and only the O atom was detected in both films. The O concentration in pulsed-MOCVD films was much lower than that in MOCVD films with a concentration ratio below 1/3.